1. Field
The invention relates to a vacuum booster.
2. Related Art
For example, Japanese Patent No.2913237 describes a vacuum booster.
In the vacuum booster, a mounting bead portion of a valve element is held by a valve element holder so as to be in close contact with an inner circumferential surface of a valve cylinder.
When an input rod retracts, a vacuum pressure in a vacuum chamber acts on a front surface of a flexible portion that connects a valve portion of the valve element and the mounting bead portion and an atmospheric pressure in an atmospheric chamber acts on a rear surface of the flexible portion. Therefore, a pressing force resulting from a differential pressure between the pressure acting on the front surface and that acting on the rear surface of the flexible portion biases the valve portion forward, which requires an input return spring to have a set load which is enough to retract the input rod against the forward pressing force.
However, in the above described holding configuration of the valve element, the valve portion and the mounting bead portion of the valve element are spaced largely away from each other in a radial direction. Therefore, the flexible portion, which connects the valve portion with the mounting bead portion, has relatively wide pressure bearing areas of the front surface and the rear surface which bear the vacuum and the atmospheric pressure, respectively. Thereby, the forward pressing force resulting from the differential pressure to act on the flexible portion becomes large. Then, the load set on the input return spring has to be relatively large. However, in the event that a large load is set on the input return spring, this naturally increases the operation force of the input rod when the input rod is advanced, which is not preferable from the viewpoint of keeping a pedal operation feel good.